Use Seaming Tape
Position pond liners on top of each other, overlapping at least 4 inches; mark the location.
Clean all liner surfaces with acetone or a similar solvent where the Seaming Tape is to be applied.
Align Seaming Tape on bottom liner, 1 inch beyond the marked line. Unroll the tape along the entire length of the seam.
Before removing the backing, apply pressure along the entire length of seaming tape, ensuring maximum hold.
Place top liner over seaming tape. Slide backing out from under top liner. Apply pressure along entire length of seam to seal properly.